1. Field of the Invention
The present invention relates to a double-side display device and surface light source device.
2. Description of the Related Art
FIG. 1 is a schematic sectional view showing an example of a conventional double-side display device. A double-side display device 11 includes a reversible light 12 and a semi-transmissive liquid crystal panel 13. The reversible light 12 is a surface light source device having function of uniformly emitting light and transparency, and the reversible light 12 is also a surface light source device having both back light function and front light function. The reversible light 12 is arranged while facing the semi-transmissive liquid crystal panel 13. The reversible light 12 is one in which a light source 15 such as a cold-cathode tube is arranged while facing an end face (light incident plane) of a light guide plate 14. A fine corrugated surface 17 is formed in the opposite surface to a light outgoing plane 16 of the light guide plate 14. In the following description, the opposite side to the semi-transmissive liquid crystal panel 13 with respect to the reversible light 12 is referred to as front light side, and the opposite side to the reversible light 12 with respect to the semi-transmissive liquid crystal panel 13 is referred to as back light side. A light beam is indicated by an arrow, and noise light is indicated by a broken-line arrow except as otherwise noted.
In the double-side display device 11, as shown in FIG. 1, the light emitted from the light source 15 is incident into the light guide plate 14 from the end face of the light guide plate 14, the light propagates through the light guide plate 14 by repeating total reflection at the light outgoing plane 16 and its opposite surface of the light guide plate 14, and the light spreads in the whole of the light guide plate 14. At this point, in the light scattered by a fine corrugated surface 17, the light incident to the light outgoing plane 16 with a small incident angle is transmitted through the light outgoing plane 16 and outputted to the outside from the reversible light 12.
Part of the light outputted from the reversible light 12 is transmitted through a transmission region of the semi-transmissive liquid crystal panel 13 and outputted to the back light side. Part of the light outputted from the reversible light 12 is reflected by a reflecting region of the semi-transmissive liquid crystal panel 13 and outputted to the front light side. Therefore, when viewed from the back light side, the reversible light 12 function as the back light so as to visually recognize an image generated by the semi-transmissive liquid crystal panel 13. When viewed from the front light side, the reversible light 12 function as the front light so as to visually recognize the image generated by the semi-transmissive liquid crystal panel 13.
In the double-side display device having the above configuration, because each one of the reversible light 12 and the semi-transmissive liquid crystal panel 13 is required, the number of components is decreased, which leads to merits such as inexpensive production of the double-side display device 11, a low profile, and weight reduction. However, in the double-side display device 11, there is a problem that contrast is low due to the noise light when viewed from the front light side.
FIG. 2 is a view for explaining the reason why the contrast is low on the front light side in the conventional double-side display device. Assuming that a light quantity of the light incident into the light guide plate 14 from the light source 15 is set at “100”, in the light reflected toward the light out going plane 16 by the fine corrugated surface 17, the light quantity of “0.5” is reflected by the light outgoing plane 16 and outputted to the front light side, and the light quantity of “0.5” is reflected by the surface of the semi-transmissive liquid crystal panel 13 and outputted to the front light side in the light quantity of “99.5” of the light outputted from the light outgoing plane 16 of reversible light 12. Accordingly, the light having the light quantity of “99” is incident into the semi-transmissive liquid crystal panel 13, and only the light quantity of “6.4” is reflected by the inside of the semi-transmissive liquid crystal panel 13 and outputted in the form of signal light (image) to the front light side. Similarly only the light quantity of “6.4” is transmitted through the semi-transmissive liquid crystal panel 13 and outputted in the form of signal light (image) to the back light side. Therefore, on the front light side, the noise light is “0.5”+“0.5”=“1.0” to the signal light of “6.4” which becomes the image, and the image contrast is 6.4:1 (hereinafter light quantity ratio of the signal light to the noise light is referred to as contrast ratio). Thus, since the proportion of the noise light is large, the image contrast is low, and the image quality is worse on the front light side.
FIG. 3 shows a method of improving the contrast on the front light side. In the double-side display device 11, the light is outputted from the reversible light 12 while inclined from a direction perpendicular to the light outgoing plane 16, and the light is reflected toward the direction perpendicular to the light outgoing plane 16 by the reflecting region of the semi-transmissive liquid crystal panel 13. On the front light side, while the signal light is outputted toward the direction perpendicular to the double-side display device 11, the noise light which is normally reflected from the surfaces of the light outgoing plane 16 and the semi-transmissive liquid crystal panel 13 is outputted toward the oblique direction. Therefore, the noise light indicated by the broken-line arrow is not incident to eyes to suppress the decrease in contrast when the double-side display device 11 is viewed from the front face on the front light side.
However, in the double-side display device 11 having the above structure, when an outgoing peak angle θ is small in the light outputted from the light outgoing plane 16 of the reversible light 12, the contrast is worsened on the front light side by the reason described in FIG. 2 (see FIG. 2). The outgoing peak angle θ is an angle formed by a normal perpendicular to the light outgoing plane 16 and a peak brightness direction in which brightness of the light outputted from the light outgoing plane 16 is maximized. As shown in FIG. 4, when the outgoing peak angle θ is excessively large, front face brightness is decreased to be hardly visible from the front face on the back light side.
Accordingly, for the conventional double-side display device, it is demanded that the good-look image is obtained when the double-side display device is viewed from both the front light side and the back light side, and particularly it is necessary to improve the contrast on the front light side and the brightness on the back light side.